The invention relates to a process for the manufacture of substrates from carbon-coated silicon dioxide fabrics that can be used for large-surface silicon bodies, such as those used for the production of solar cells according to a strip-coating process.
The use of monocrystalline silicon for the construction of solar cells is the determinative factor in calculating the overall manufacturing costs for the solar cells. Polycrystalline silicon is well-suited for the manufacture of solar cells with a satisfactory degree of efficiency (10% and up) and significantly lower costs.
U.S. Pat. No. 4,171,991 discloses a process for the strip drawing of silicon bodies for solar cells. In this process a graphite cloth strip containing perforations is dipped into a silicon melt and coated with silicon by a continuous drawing process. This process requires substantial quantities of silicon melt and the use of expensive graphite strips.
Published German Patent Application DE-OS No. 3,010,557 discloses a strip-coating process for silicon, in which a carrier body consisting of graphite threads with a net-like structure is used. The coating is conducted, with reference to the drawing speed, so that as a result of the high surface tension of the molten silicon, only a thin silicon layer is formed in the meshes of the net. Then, after hardening, the network consisting of the graphite threads is integrated into the silicon body. This process is more cost-efficient than the process described in U.S. Pat. No. 4,171,991, because only minimal quantities of silicon are consumed. In place of the graphite threads, it is possible to use graphitized silica glass fibers for the net-like structure used in published German Patent Application DE-OS No. 30 10 557. This is advantageous because these fibers are substantially cheaper than the graphite threads.
The substrate for the coating must be a material that neither reacts with the molten silicon, nor forms a eutectic mixture with it. Furthermore, the substrate for the coating should not soften at high temperatures, so that the shape of the substrate remains unaltered despite changes in temperature.
These requirements are met by SiO.sub.2 in a highly pure form. The use of high-purity SiO.sub.2 for the manufacture of sheet silicon requires SiO.sub.2 in the form of high-purity SiO.sub.2 fibers and fabrics. Both of these forms of SiO.sub.2 are commercially available, but there is no way to use them economically, because of their excessively high price.